The present invention relates to image filtering, and more particularly, to systems for and methods of spatial image filtering, wherein the filter response characteristics may be modified to meet the needs of the user.
“Level and Window” concept—In digital radiography (DR), an acquired image may be characterized by a dynamic range that is significantly larger than the display system or the ability of the person viewing the image to resolve the image. Consider an acquired image with an exemplary dynamic range of 12 bits, i.e., 2048 intensity states. The resolution of the human eye is limited under the best conditions to only a few hundred grayscale levels. Typically, the human eye is limited to less than one hundred grayscale levels. The level and window concept maps a portion of the acquired dynamic range into a display system. For example, a display may have a dynamic range of 8 bits, or 256 grayscale states. Using the level and window concept, the display would then map any arbitrary portion of the overall image dynamic range. The user may select which portion of the acquired image dynamic range is to be mapped to the display, or the range may be “swept” across the entire dynamic range while the user observes the display. The ability to view different portions of the dynamic range is useful to the user because certain aspects of the image may only be apparent in particular portions of the dynamic range, or they may be much more pronounced in those portions of the dynamic range.
Subjects of x-ray images have varying degrees of contrast and physical size. One category of subjects (e.g., masses) are characterized by large physical size in the image (with respect to surrounding and overlapping structures that are of no diagnostic interest), but may have relatively constant contrast across the object (i.e., homogenous with respect to intensity level). These subjects may not be easy to discern within the x-ray image when the subject overlaps an object with high contrast characteristics. These subjects are characterized by relatively low spatial frequency components in the image, due to their physical size and constant contrast characteristics. Low-pass filtering of these subjects can therefore be used to differentiate them from overlapping objects having high contrast characteristics.
Another category of subjects (e.g., instances of micro-calcification) has high contrast, but are characterized by physically small details that make them difficult to detect, regardless of the background. These subjects are generally characterized by wide-band and/or high spatial frequency components in the image. High-pass filtering of these subjects helps to discern the subjects from other objects by increasing the contrast of the subject with respect to surrounding and overlapping structures. Due to the wide-band nature of some of these subjects, however, a high-pass filter may also reduce useful information regarding the subjects.
Prior art image filtering systems typically provide a fixed frequency response at a given point in time, depending upon the particular application for which the filter is being used. Some prior art image filters provide adaptive capabilities to optimize signal to noise characteristics of the image, but such filters are typically adaptive only to components of the image itself. One disadvantage to such prior art filtering systems is that a single, fixed frequency response filter can not clearly show all aspects of a particular image, as described herein.
It is an object of the present invention to substantially overcome the above-identified disadvantages and drawbacks of the prior art.